The present invention relates to a system for deforming a turbine stator housing, particularly such a system utilized in conjunction with a turbojet engine to maintain radial clearance between the stator and a rotor whose longitudinal axis may undergo radial deformation.
Studies conducted to improve the performance of turbojet engines have lead engine designers to find ways to optimize the radial clearances between the rotor and the stator of the turbojet engine. The maintenance of a minimum, but adequate, radial clearance between the rotor and the stator under all operating conditions directly effects the efficiency of the turbojet engine, the maximum thrust of the engine as well as minimizes the conditions under which the surging phenomena occurs.
Various proposals have been tried over the years to maintain this radial clearance under all engine operating conditions. The radial expansion of the turbine rotor, which is acted upon by extremely hot gases emanating from the turbojet engine combustion chamber, is effected by the temperatures of the rotor blades and the rotor wheel, as well as the speed of rotation of the rotor. The relative difference in the dimensions between the rotor blades and the rotor wheel causes these elements to expand at different rates. Thus, it can be seen that the radial clearance between the rotor and the stator is effected by the relative temperatures of these elements, as well as the rotational speed of the rotor, which varies throughout the operational envelope of the turbojet engine.
It is known to bleed air from one or more stages of the turbine compressor, or from the combustion chamber, and to direct this air onto the stator or rotors to adjust the temperature of these elements. The temperature of the air may be modulated by mixing the air from different sources, such as the turbine stage or the combustion chamber.
Typical examples of such systems can be found in U.S. Pat. Nos. 4,419,044; 4,304,093; and 4,230,439 as well as U.K. Patents 1,581,855 and 1,581,566. In these systems, the control of the bleed airflow regulators is based upon a sensor measurement of the magnitude of such items as temperature, rotational speed of the rotor, or a direct measurement of the radial clearance at any given instant. In some cases, a hydromechanical regulator controls the opening of the airflow regulating valves based upon a pre-established control program. While these systems have been theoretically feasible, as a practical matter, they have been found to be somewhat limited in maintaining the concentricity of the radial clearance between the rotor and the stator when an irregular distribution of the peripheral radial clearance exists.
Solutions have also been proposed to verify the correct centering between the rotor and the stator and to make corrections of the observed flaws. Typical examples of these solutions involving mechanical adjustment can be found in U.S. Pat. Nos. 4,222,708 and 4,548,546. Again, these solutions have proven to be inadequate for solving certain off-centering problems occurring during operation between the rotor and the stator wherein the excessive clearances between the rotor and the stator are unequally distributed about the peripheries. This has caused the reduction of the efficiency of the turbojet engine due to the disturbances caused in the aerodynamic gas flow by these peripheral distribution clearances.
The known systems have been found to be particularly deficient under certain operating conditions of the turbojet engine, particularly under transient conditions, in which the rotary and stationary parts of the turbojet engine may come into contact with each other on certain peripherally limited zones. Such contact causes excessive turbine blade wear which, in turn, creates excessive radial clearances, thereby reducing the efficiency of the turbojet engine. It has been found that this contact between the rotor and the stator on such peripherally limited zones is caused by radial deformation of the longitudinal axis of the rotor. As illustrated in FIG. 1, the radial displacement of the rotor axis R may be caused by worn bearings and/or the bending of the rotor shaft such that it assumes the bowed position illustrated in this figure. During such operation, the longitudinal axis of the stator S1 does not undergo corresponding deformation, thereby decreasing the radial clearance at a certain portion of the periphery and possibly causing contact between the elements at this portion.
It has been observed that, especially in aeronautical applications of the turbojet engine, that a more rapid cooling of the lower portion of the stator housing takes place during engine shutdown which results in a deformation of the longitudinal axis of the stator to assume the position S2 illustrated in FIG. 1. This stator housing deformation increases the possibility of contact between the rotor and the stator during this portion of the engine's operation.